True magnetic structure of the ferrimagnetic garnet Y3Fe5O12 and magnetic moments of iron ions

To examine and emphasize the difference between approximative and true magn etic structure, we choose ferrimagnetic garnet Y3Fe5O12 whose magnetic prop erties are well known. In order to study the magnetic structure, neutron di ffraction experiments were done on powder sample of Y3Fe5O12 at 10 and 295 K at zero field and at 295 K in the applied held B = 0.2 T. Using these dat a the crystal and magnetic structures were first refined in the space group Ia3d. Beside the use of a cubic space group, for a magnetically ordered cr ystal, this solution suffers from unrealistic magnetic moment per formula u nit, which are all significantly above 5 mu(B) and temperature factors at 1 0 K which lead to unreliable Debye temperature. To satisfy compatibility of the symmetry of the magnetic moments (oriented along the body diagonal in cubic system) and crystal symmetry, the crystal and magnetic structures wer e also refined in the trigonal space group R (3) over bar. In this group th e magnetic moments orientation, along the principal axis, is compatible wit h the symmetry of local positions. The magnetic R factors are significantly lower than in cubic system. The magnetic moment per formula unit: 3.1 mu(B ), from the diffraction measurements in the field, is close to the result o btained by classical measurements of magnetization. (C) 1999 Elsevier Scien ce B.V. All rights reserved.